The subject of the invention is a method for improving the separating action in the ultrafiltration of aqueous solutions, particularly cleaning solutions, in order to be able to remove organic substances, like oils, fats, surface-active compounds or tensides and emulsifiers.
Ultrafiltration leads to the separation of high-molecular-weight organic substances, dispersed or dissolved in the aqueous phase, from low-molecular-weight substances. In this method, the aqueous phase is carried along an asymmetrical membrane or diaphragm under pressure. Water and the low-molecular-weight substances dissolved therein both inorganic and organic, pass through the membrane. The undissolved and dissolved high-molecular-weight organic substances accumulate in the retentate. A measure of the separating power of ultrafiltration membranes is the retention power R. ##EQU1## where
Cp=the concentrate in the permeate (portion passing through the membrane)
Cr=the concentrate in the retentate (residue, portions not passing through the membrane)
The ultrafiltration method is frequently used for the treatment of waste water originating from used treatment solutions, for example, spent cleaning solutions and wash waters. Untrafiltration has also been suggested for the regeneration of degreasing solutions in metal cleaning.
Both organic and inorganic membranes or diaphragms with a retention power for molecules with a molecular-weight of between 2,000 and 40,000 can be used for the method. With a ultrafiltration it is possible to remove coarsely dispersed oils and tensides in the form of high-molecular-weight micells, as well as finely-divided solids, from the aqueous phase. Low-molecular-weight substances, particularly water-soluble salts, as they are present in the form of phosphates, silicates, caustic soda in cleaning solutions, but also water-soluble molecularly dissolved tensides and solubilized or very finely emulsified oils pass through the membrane and are found in the permeate. In sewage treatment, however, it is desirable to remove all organic substances as far as possible in order to keep the chemical oxygen demand (COD) in the sewage at a minimum.
The effectiveness of cleaning solutions is impaired not only by oils but also by certain surface-active substances, for example, petroleum sulfonates and anti-corrosives, which partially penetrate the membrane. In addition to the removal of oil, it is therefore of technical interest for the regeneration of cleaning solutions to remove all other organic substances as for as possible.
A particular problem exists in the treatment of so-called cold cleaning emulsions. Cold cleaning emulsions are mixtures of emulsifiers and solvents used to degrease metals. In their use, the contamination with the cold cleaning emulsion is rinsed off from large workpieces by means of water. Before these emulsions are introduced into the sewers, they must be freed of solvents and oils.
In the practical use of the ultrafiltration method for the regeneration of degreasing solutions and/or the treatment of contaminated waste water, the separating effect varied, depending on the degreasing agent and on the impurities in the aqueous solution.
For example, water-soluble, non-ionic tensides pass completely through the ultrafiltration membranes. Water-soluble, anionic-active tensides, are more or less retained, however, depending on the separating power of the membrane.